1. Field of the Invention
This invention relates to method and apparatus for the continuous on-site purification of ultrapure liquids, especially liquids used in a semiconductor wafer cleaning process, such as ultrapure peroxydisulfuric acid and sulfuric acid solutions.
2. Description of the Prior Art
In the past, it has been common practice in industries requiring chemicals, especially ultrapure chemicals, to utilize such chemicals until a certain degree of contamination was reached. At that point, it was necessary to remove the contaminated chemicals from the process apparatus, clean the apparatus, and add new chemicals as needed. Contaminated chemicals were commonly disposed of by any convenient means. This has included legal and illegal dumping in land areas and occasionally in waterways.
In the semiconductor industry, it is important to remove all organic and inorganic particles from the surface of semiconductor wafers. This is commonly done by immersion in an acid bath. A preferred acid bath consists of an oxidant solution of sulfuric acid and either peroxydisulfate ion, which has the formal S.sub.2 O.sub.8.sup.-2, or hydrogen peroxide and ultrapure water. The oxidant solution is commonly made by mixing together the oxidant and sulfuric acid. This combination produces a highly oxidizing compound which attacks carbon or other organic particles on the surface of the wafers.
The wafers are commonly held in a cassette boat whereby they can be cleaned by immersion into a tank containing oxidant solution. The time for immersion is usually about ten to twenty minutes. After immersion, the cassette boat containing the wafers is then washed in ultrapure water. The purity of the water is determined by measuring the resistivity of the water.
In prior art processes very high purity sulfuric acid and H.sub.2 0.sub.2 or a source of peroxydisulfate are required. The bath temperature is maintained at about 80.degree. C.-150.degree. C. In about one half hour, contamination of the acid takes place with an increased concentration of particles. At this time, the acid is normally dumped and a new bath of high purity acid is added.
In recent years, two developments have made this approach undesirable. The first of these has been the requirement of increasingly greater purity of chemicals, especially in industries such as the semiconductor and pharmaceutical industries. The second development has been an increase in concern for the environmental effects of the dumping of hazardous waste materials in the sewer lines, as well as on land.
With regard to the purity of chemicals, it is evident that the purity of a liquid over a period of time is greater at the start of a process time period than it is at the end of that time period. As greater purity has become more and more important, it has become apparent that higher quality is produced using chemicals during the first part of the period when purity is greater, than at the end of the tolerable processing period when contaminants have been able to build up in the chemical liquid. As a consequence, in the specific case of the cleaning of semiconductor wafers using peroxydisulfuric acid, the wafers cleaned at the beginning of the process period have a higher quality than those which are cleaned at the end of the tolerable contaminant processing period.
With respect to the dumping of hazardous chemicals, public awareness coupled with recently passed hazardous waste chemical disposal laws, have made the disposal of hazardous chemicals extremely difficult as well as costly.
In addition, the necessity of periodic replacement of chemically pure liquids represents an increased cost of materials, increased labor costs, as well as a small but real risk of contamination or hazard to the personnel involved. Finally, there is the cost involved in shutting down a process for whatever time is required to replace the chemicals.
In addition, any time chemicals are stored or transferred, impurities are introduced which are intolerable for ultrapure requirements. For example, stabilizers must often be added to prevent decomposition of unstable compounds. Also, reaction with the containers during storage and transfer, although slight in most cases, often produces a contamination level in such liquids which is intolerable for ultrapure process requirements.
In the case of semiconductor wafer cleaning various chemicals can be used. One process utilizes hydrogen peroxide which must be shipped with stabilizers in order to prevent spontaneous decomposition. The stabilizers which are required to be used introduce impurities which will ultimately contaminate the wafers during the cleaning process.
Another process utilizes potassium or ammonium peroxydisulfate. Potassium peroxydisulfate commonly contains metal ions as impurities which produces a known problem with integrated circuits, particularly MOS circuits.
While ammonium peroxydisulfate could theoretically be made quite pure, such purity levels are not available on an economically attractive basis.
In light of the above difficulties in requirements for the use of ultrapure chemicals and the subsequent contamination and disposal requirements, it is desirable to provide a method and apparatus capable of maintaining purity of the ultrapure liquid throughout the course of the reaction which will avoid contamination buildup. In addition, it is desirable to provide a process and apparatus which avoid the need for the disposal of large amounts of hazardous chemicals. Finally, it is desirable to provide a process and apparatus which reduce processing costs by reducing the amount of chemicals required, reducing the number of personnel involved, increasing the safety of the personnel involved, and eliminating the frequent requirements for shutdown of the process for purposes of renewing ultrapure liquids.